The use of multilayer microporous membranes as battery separators for lithium ion secondary batteries is known, for example, see patents U.S. Pat. Nos. 5,480,745, 91,047, 5,667,911, 5,691,077 and 5,952,120, each of which is incorporated herein by reference. These patents disclose monolayer and multilayer membranes which are comprised of one or more polyolefin layers made using a solvent free manufacturing process commonly known as the dry process. One such well known dry process is the Celgard® dry process which involves the formation of a film parison using either a co-extrusion die or a non-coextrusion die. This film parison is also known as a precursor membrane and has a nonporous microstructure.
Dry process multilayer separator membranes such as Celgard® separators made by Celgard LLC of Charlotte, N.C. have a unique microstructure, defined by pore size, porosity and tortuosity and inhibit dendrite growth and improve the cycle life of a lithium ion secondary rechargeable battery. See FIG. 1.
Microporous separator membranes can also be made by a wet process such as those described in U.S. Pat. Nos. 4,588,633, 4,600,633, 4,620,955, 4,539,256, 5,922,492 and US 2009/0253032, each of which is incorporated herein by reference, which disclose monolayer and multilayer membranes made using a manufacturing process which includes the use of a solvent. The wet process involves a phase inversion process such as, for example, a thermal phase inversion process (TIPs) where a polymeric material is combined with a process oil or a plasticizer to form a mixture which is extruded and cooled on a cast roll to form a precursor membrane. Pores are then formed in the precursor membrane when the membrane is stretched and the plasticizer is extracted or removed (these films may be stretched before or after the removal of the oil). The extraction step involves the use of a solvent, thus the origin of the name ‘wet process’. Wet process separators have an anisotropic, non-oriented pore structure which is web-like in appearance under Scanning electron microscope (SEM). See FIG. 2. Wet process Polyethylene (PE) microporous separator membranes are made using a high molecular weight PE, typically 100,000 to 500,000, and/or an ultrahigh molecular weight PE, typically with a molecular weight>500,000, and sometimes with a molecular weight≧1 million. Membranes made using the wet process are typically biaxially stretched and have high MD and TD tensile strength.
Microporous separator membranes made by dry process and the wet process have unique separator performance properties due to their different manufacturing methods. Microporous separator membranes made by dry process or the wet processes are both commonly used today as battery separators in lithium ion secondary rechargeable batteries.
There is a need for a new separator membrane combines the performance properties of dry process microporous membrane with that of a wet process microporous membrane to produce a superior performance separator membrane with balanced MD and TD tensile strength, high overall tensile strength, high puncture strength, good oxidation resistance and a shutdown function.